1. Field
The disclosed concept relates generally to circuit interrupters, and, in particular, to a circuit interrupter that employs a linear transducer, such as, without limitation, a linear variable differential transformer (LVDT), to monitor contact erosion.
2. Background Information
Circuit interrupters provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits and abnormal level voltage conditions. Typically, circuit interrupters include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors of an electrical system in response to abnormal conditions, although a wide range of driving mechanisms may be employed.
Circuit interrupters, such as, for example, power circuit breakers for systems operating above about 1,000 volts, typically utilize vacuum interrupters (VIs) as the switching devices. Vacuum interrupters include separable main contacts disposed within an insulating housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor which is interconnected with the power circuit associated with the circuit interrupter. The other contact is moveable. In the case of a vacuum interrupter, the moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within a vacuum chamber and a driving mechanism coupled at the other end which is external to the vacuum chamber. An operating rod assembly comprising a push rod, which is fastened to the end of the stem opposite the moveable contact, and a driving mechanism provide the motive force to move the moveable contact into or out of engagement with the fixed contact.
VIs are typically used, for instance, to reliably interrupt medium voltage alternating current (AC) currents and, also, high voltage AC currents of several thousands of amperes or more. Typically, one VI is provided for each phase of a multi-phase circuit and the VIs for the several phases are actuated simultaneously by a common operating mechanism, or separately by separate operating mechanisms (and separate auxiliary switches).
Over the life of a VI, the surface of each of the contacts will slowly erode due to the arcing that occurs between the contacts during each interruption. Typically, each contact will lose 1-1.5 mm of material over its life due to erosion. This loss of material is taken up by the contact spring, causing it to lose 2-3 mm of its compression. This loss of compression translates to a loss of compressive force on the contacts within the VI. Sufficient contact force is very important to the performance of VIs. In particular, insufficient contact force increases the risk of overheating or explosion in the event of a short circuit. Thus, it is important to monitor the amount of contact erosion over the life of a VI in order to monitor the extent to which contact force is decreasing. Such monitoring, however, is difficult or even impossible in current designs where visual access to the contacts is limited or nonexistent (e.g., designs where the contacts are hidden).
There is thus a need for a mechanism for monitoring the amount of contact erosion over the life of a VI that will be operable even in designs where visual access to the contacts is limited or nonexistent.